The invention relates to novel polycarbosilanes and to processes for preparing them.
Polycarbosilanes are polymers having a backbone structure made from the elements carbon and silicon, in which in general Si groups and hydrocarbon groups are present alternately. The backbone structure of polycarbosilanes of this type consists, for example, of repeating structural units corresponding to the formula ##STR1## wherein R.sup.0 represents, for example, a hydrocarbon substituent. According to known preparation processes polycarbosilanes of this type are obtained by thermally decomposing monosilanes, such as for example tetramethylsilane, trimethylchlorosilane, dimethyldichlorosilane or methyltrichlorosilane, to convert them into mixtures of various polycarbosilanes. A further known process for preparing polycarbosilanes of this type starts from polysilanes in which at least one of the two substituents on the silicon atom is a methyl group. These polysilanes are converted to the polycarbosilane by pyrolysis at temperatures of 350.degree. to 450.degree. C. During the pyrolysis or thermal conversion, methylene groups are formed from some of the methyl substituents and are inserted between adjacent Si atoms of the polysilane, and a hydrogen atom remains on the silicon atom. Pyrolyses of this type proceed through a free-radical reaction mechanism.
Polycarbosilanes, in which the Si atoms are linked by bridges of organic aromatic groups, or preferably heteroaromatic groups, such as pyrrol-2,5-diyl or thiophen-2,5-diyl, are known from German published application No. 36 34 281. The object of this application is to prepare conductive polysilanes by additional chemical or electrochemical doping.
It is known from British patent specification No. GB 896,301 to convert monomeric diaryldihalosilanes and p-phenylene dimagnesium bromide via a Grignard reaction to polycarbosilanes in which the Si atoms are linked by phenylene bridges. Thermoplastic, heat-curable resins are obtained.
It is known to prepare copolymers made from silane monomer units and olefin units in the presence of potassium in tetrahydrofuran in accordance with Schilling and Williams (Schilling, C. L., Jr.; Williams, T. C. (Union Carbide Corp., Tarrytown, N.Y., USA). Report 1983, TR-83-2; Order No. AD-A141558, 15 pp. (Eng). Avail. NTIS. From Gov. Rep. Announce. Index (U.S.) 1984, 84(18), 48; see also Chemical Abstracts 101:196821q). Methyltrichlorosilane, dimethyldichlorosilane or methyldichlorohydrosilane are reacted as silane monomers with styrene or isoprene, wherein in the case of styrene, the Si units are linked by phenyl-substituted ethylene units. In the case of isoprene, the Si units are linked by the corresponding methyl-substituted C.sub.4 -alkylene chain which has a further double bond. In two additional examples, isoprene is reacted with methylchloro-methyldichlorosilane or with a mixture of vinylmethyldichlorosilane and trimethylchlorosilane.
Halogenated polycarbosilanes are known from U.S. Pat. No. 4,761,458. These halogenated polycarbosilanes are prepared from polycarbosilanes which carry at least 0.1 wt % of SiH groups and which are converted to chlorinated or brominated polycarbosilanes by reacting with chlorinating or brominating reagents in a free-radical reaction, whereby SiCl or SiBr groups are formed from the SiH groups. As educts for the halogenation reaction, U.S. Pat. No. 4,761,458 uses conventional polycarbosilanes of the type described above which are substituted by lower alkyl groups and are known from the state of the art. These were prepared by pyrolysis of, for example, polydimethylsilane (--(CH.sub.3).sub.2 Si--).sub.n.
Furthermore, prepolymers made from ceramic-forming elements for the preparation of ceramic polymer materials are known from German published application no. DE 36 16 378. In these compounds easily cleavable elements are partly replaced by elements which are difficult to cleave, such as fluorine or completely fluorinated hydrocarbon groups. Hydrogen is mentioned there as an easily cleavable element. The exemplary embodiment of the published German application also starts from a conventional polycarbosilane of the type described above, which is known from the state of the art and was prepared by pyrolysis of polydimethylsilane (--(CH.sub.3).sub.2 Si--).sub.n. Fluorine is introduced into this polycarbosilane by electrofluorination using tetraethylammonium fluoride or by direct (free radical) fluorination using elemental fluorine. In this case, in addition to the conversion of SiH groups into SiF groups, fluorine atoms are also introduced into the methyl substituents on the silicon atoms and into the methylene bridges of the Si--CH.sub.2 --Si backbone of the polycarbosilane.
Indeed, a number of polycarbosilanes and also some halogenated polycarbosilanes, and processes for their preparation, are already known in the state of the art. Yet certain types of polycarbosilane could not be prepared in the prior art. For example, it has not heretofore been possible to prepare polycarbosilanes in which the Si atoms are linked by phenyl-substituted methylene bridges or by defined aliphatic hydrocarbon bridges partially or completely substituted by fluorine. Likewise, it has not heretofore been possible to prepare polycarbosilanes which have a defined structure and carry fluorine substituents on the Si atoms, and in which the Si atoms carry linking, aliphatic hydrocarbon bridges.
Furthermore, the polycarbosilanes known from the state of the art, in particular those obtained pyrolytically, and halogenated polycarbosilanes prepared therefrom by free radical halogenation, are subject to a series of disadvantages with regard to the properties of the products and the processes by which they are prepared. The disadvantageous properties of these known polycarbosilanes are attributable to the unfavorable effects of their pyrolytic preparation, by means of which the basic structure and the maximum attainable degree of purity for the polycarbosilanes and also the halogenated derivative products is already essentially predetermined. Hence the known halogenated and non-halogenated polycarbosilanes are non-uniform products which have an irregular SiC backbone and are accompanied by more or less volatile decomposition products, the identity of which depends on the preparation method. However, additional measures for limiting the products to a product spectrum more favorable for the intended further use (for example purification and/or separation by fractional crystallization or fractional distillation) are work, energy and cost intensive. The ease with which halogen atoms are introduced into the polycarbosilanes of the prior art also depends directly on the presence of SiH groups in these polycarbosilanes, since only these SiH groups may be converted to SiHal groups (Hal=halogen) using the known processes for preparing halogenated polycarbosilanes. However, the formation of SiH groups in the pyrolytic preparation of polycarbosilanes is difficult to control, and this also has a direct effect on the properties of halogenated polycarbosilanes prepared therefrom. Furthermore, the degree and location of halogenation also is difficult to control in the preparation of known halogenated polycarbosilanes which is carried out under free radical halogenation conditions. Indeed, the SiH groups preferably react initially to form SiHal groups, yet there are considerable side reactions, particularly when long reaction times and slightly intensified reaction conditions are used. Hence, in addition to the required halogenation in the SiH groups, nonspecific halogenation reactions also occur in the hydrocarbon substituents, at silicon atoms, or in the methylene bridges of the polycarbosilane used. Furthermore, the free radical reaction conditions may lead to splitting reactions in the Si-C-Si backbone of the polycarbosilane, as a result of which the polycarbosilane and/or the halogenated polycarbosilane which is used may be partly degraded into undesirable fragments and volatile, low molecular weight compounds during the reaction.